Synthesis gas comprises hydrogen and carbon oxides (carbon monoxide and carbon dioxide) and may contain nitrogen and other inerts such as argon and low levels of methane. The synthesis gas may contain greater or lesser amounts of hydrogen and carbon oxides suited to the particular end use such as hydrogen manufacture for refineries or fuel cells, ammonia synthesis, methanol synthesis, dimethylether synthesis or the Fischer-Tropsch process for the synthesis of liquid hydrocarbons
In a steam reforming process a process fluid, i.e. a mixture of a hydrocarbon feedstock and steam, and in some cases also carbon dioxide or other components, is passed at an elevated pressure through catalyst-filled heat exchange tubes, which are externally heated by means of a suitable heating medium, generally a hot gas mixture. The catalyst is normally in the form of shaped units, e.g. cylinders having a plurality of through holes, is typically formed from a refractory support material e.g. alumina, impregnated with a suitable catalytically active metal such as nickel.
The steam reforming reactions are endothermic and heat must be supplied to the gas undergoing reforming. The heat may be provided by combustion gases e.g. combusted methane, in a combustion furnace reformer or by ‘externally-heated’ hot gases, for example a flue-gas. Alternatively, the catalyst-filled tubes may be externally heated by means of the process gas that has passed through the tubes but which has then been subjected to further processing before being used as the heat exchange medium. The further processing step advantageously includes a step of partial oxidation with an oxygen-containing gas, which both provides further conversion of hydrocarbon feedstock and heats the reformed gas mixture. For example, primary catalytic steam reforming may be effected in a heat exchange reformer in which the catalyst-containing reformer tubes are heated by a secondary reformed gas. Examples of such reformers and processes utilising the same are disclosed in for example GB1578270.
During the commissioning and operation of steam reformers catalyst damage can occur. The damage may be caused by a number of reasons, for example, during catalyst loading, tube vibration, start up and shut down thermal cycling, carbon formation and wetting. The damage so-caused ranges from attrition of the catalyst surfaces, forming dust particles, to catalyst breakage and disintegration. Catalyst damage, if not severe, generally does not cause immediate problems and the reformer continues to operate. However, as the reformer tubes are typically vertical and the flow direction of the gas undergoing reforming is usually downwards, the catalyst fragments and dust generated from the catalyst damage can work their way down to the bottom of the tube.
The catalyst not only acts as the surface for the reforming reaction, but its presence in the tube also acts to enhance heat transfer by increasing turbulence in the flow of process fluid within the tube. We have found that if the catalyst is damaged and a sufficient amount of small fragments of catalyst are present at the bottom of the tube then the heat transfer from the tube wall to the gas undergoing reforming will reduce. As the tubes are being heated from an external source, the tube wall temperature will increase. The pressure drop through the catalyst will also increase and this additional hydraulic load will be transferred to the tube wall thereby increasing the stress. The resulting deformation, also known as creep, will ultimately cause the tube to rupture. Thus, we have found that catalyst breakage and accumulation in the bottom portion of the heat exchange tubes can result in tube failure sooner than expected.